task_for_pid() / memory_maps on OSX may raise ENOENT

[giampaolo]

See: https://travis-ci.org/giampaolo/psutil/jobs/303824997.
Error originates from task_for_pid(MACH_PORT_NULL) call.

======================================================================
ERROR: psutil.tests.test_misc.TestScripts.test_procinfo
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Traceback (most recent call last):
  File "/Users/travis/build/giampaolo/psutil/psutil/tests/test_misc.py", line 707, in test_procinfo
    self.assert_stdout('procinfo.py', args=str(os.getpid()))
  File "/Users/travis/build/giampaolo/psutil/psutil/tests/test_misc.py", line 659, in assert_stdout
    out = sh(sys.executable + ' ' + exe, **kwds).strip()
  File "/Users/travis/build/giampaolo/psutil/psutil/tests/__init__.py", line 275, in wrapper
    return fun(*args, **kwargs)
  File "/Users/travis/build/giampaolo/psutil/psutil/tests/__init__.py", line 421, in sh
    raise RuntimeError(stderr)
RuntimeError: Traceback (most recent call last):
  File "/Users/travis/build/giampaolo/psutil/scripts/procinfo.py", line 339, in <module>
    sys.exit(main())
  File "/Users/travis/build/giampaolo/psutil/scripts/procinfo.py", line 335, in main
    run(args.pid, args.verbose)
  File "/Users/travis/build/giampaolo/psutil/scripts/procinfo.py", line 151, in run
    pinfo = proc.as_dict(ad_value=ACCESS_DENIED)
  File "/Users/travis/build/giampaolo/psutil/psutil/__init__.py", line 485, in as_dict
    ret = meth()
  File "/Users/travis/build/giampaolo/psutil/psutil/__init__.py", line 1096, in memory_maps
    it = self._proc.memory_maps()
  File "/Users/travis/build/giampaolo/psutil/psutil/_psosx.py", line 292, in wrapper
    return fun(self, *args, **kwargs)
  File "/Users/travis/build/giampaolo/psutil/psutil/_psosx.py", line 533, in memory_maps
    return cext.proc_memory_maps(self.pid)
FileNotFoundError: [Errno 2] No such file or directory